In the field of semiconductor technology, research and development efforts are continued for further miniaturization of pattern features. Recently, as advances including miniaturization of circuit patterns, thinning of interconnect patterns and miniaturization of contact hole patterns for connection between cell-constituting layers are in progress to comply with higher integration density of LSIs, there is an increasing demand for the micropatterning technology. Accordingly, in conjunction with the technology for manufacturing photomasks used in the exposure step of the photolithographic microfabrication process, it is desired to have a technique of forming a more fine and accurate circuit pattern or mask pattern.
In general, reduction projection is employed when patterns are formed on semiconductor substrates by photolithography. Thus the size of pattern features formed on a photomask is about 4 times the size of pattern features formed on a semiconductor substrate. In the current photolithography technology, the size of circuit patterns printed is significantly smaller than the wavelength of light used for exposure. Therefore, if a photomask pattern is formed simply by multiplying the size of circuit pattern 4 times, the desired pattern is not transferred to a resist film on a semiconductor substrate due to optical interference and other effects during exposure.
Sometimes, optical interference and other effects during exposure are mitigated by forming the pattern on a photomask to a more complex shape than the actual circuit pattern. Such a complex pattern shape may be designed, for example, by incorporating optical proximity correction (OPC) into the actual circuit pattern. Also, attempts are made to apply the resolution enhancement technology (RET) such as modified illumination, immersion lithography or double exposure (or double patterning) lithography, to meet the demand for miniaturization and higher accuracy of patterns.
The phase shift method is used as one of the RET. The phase shift method is by forming a pattern of film capable of phase reversal of approximately 180 degrees on a photomask, such that contrast may be improved by utilizing optical interference. One of the photomasks adapted for the phase shift method is a halftone phase shift photomask. Typically, the halftone phase shift photomask includes a substrate of quartz or similar material which is transparent to exposure light, and a photomask pattern of halftone phase shift film formed on the substrate, capable of providing a phase shift of approximately 180 degrees and having an insufficient transmittance to contribute to pattern formation. As the halftone phase shift photomask, Patent Document 1 proposes a photomask having a halftone phase shift film of molybdenum silicide oxide (MoSiO) or molybdenum silicide oxynitride (MoSiON).
For the purpose of forming finer images by photolithography, light of shorter wavelength is used as the light source. In the currently most advanced stage of lithography process, the exposure light source has made a transition from KrF excimer laser (248 nm) to ArF excimer laser (193 nm). The lithography using ArF excimer laser light of greater energy was found to cause damages to the mask, which were not observed with KrF excimer laser light. One problem is that on continuous use of the photomask, foreign matter-like growth defects form on the photomask. These growth defects are also known as “haze”. The source of haze formation was formerly believed to reside in the growth of ammonium sulfate crystals on the mask pattern surface. It is currently believed that organic matter participates in haze formation as well.
Some approaches are known to overcome the haze problem. With respect to the growth defects formed on the photomask upon long-term irradiation of ArF excimer laser light, for example, Patent Document 2 describes that if the photomask is cleaned at a predetermined stage, then it can be continuously used.
As the exposure dose of ArF excimer laser light irradiated for pattern transfer increases, the photomask is given damage different from haze; and the line width of the mask pattern changes in accordance with the cumulative irradiation energy dose, as reported in Non-Patent Document 1. This problem is that as the cumulative irradiation energy dose increases during long-term irradiation of ArF excimer laser light, a layer of a substance which is considered to be an oxide of the pattern material grows outside the film pattern, whereby the pattern width changes. It is also reported that the mask once damaged cannot be restored by cleaning with AMP (aqueous ammonia/hydrogen peroxide) as used in the above-mentioned haze removal or with SPM (sulfuric acid/hydrogen peroxide). It is believed that the damage source is utterly different.
Non-Patent Document 1 points out that upon exposure of a circuit pattern through a halftone phase shift photomask which is the mask technology useful in expanding the depth of focus, substantial degradation is induced by pattern size variation resulting from alteration of a transition metal/silicon base material film such as MoSi base material film by irradiation of ArF excimer laser light (this degradation is referred to as “pattern size variation degradation”). Then, in order to use an expensive photomask over a long period of time, it is necessary to address the pattern size variation degradation by irradiation of ArF excimer laser light.
The pattern size variation degradation by irradiation of ArF excimer laser light scarcely occurs when light is irradiated in a dry air atmosphere, as reported in Non-Patent Document 1. Then the exposure in dry air is considered a new countermeasure for preventing pattern size variation degradation. For control in a dry air atmosphere, however, extra equipment and an electrostatic countermeasure are newly needed, inviting a cost increase. It is thus necessary to enable long-term exposure in a common atmosphere (e.g., humidity ˜50%) without a need for complete removal of moisture.
Of the photomasks used in the lithography using ArF excimer laser light as the energy source, the halftone phase shift photomasks generally use transition metal/silicon base materials, typically molybdenum-containing silicon base materials. The transition metal/silicon base materials are composed mainly of transition metal and silicon and optionally contain light elements such as nitrogen and/or oxygen (e.g., Patent Document 1) and traces of carbon and hydrogen. As the transition metal, molybdenum, zirconium, tantalum, tungsten and titanium are generally used. Typically, molybdenum is used (see Patent Document 1), and sometimes, a second transition metal is added (see Patent Document 3). Also in the light-shielding film, the transition metal/silicon base materials, typically molybdenum-containing silicon base materials are used.
However, when the photomask of such transition metal/silicon base material is exposed to a large dose of high-energy radiation, a substantial pattern size variation degradation occurs by irradiation of high-energy radiation, suggesting that the lifetime of the photomask becomes shorter than the requirement. It is a serious problem that when the photomask pattern of a transition metal/silicon base material film is exposed to ArF excimer laser radiation, the photomask pattern for exposure experiences a change of line width.